Operating system: Windows 10 Pro 1903
Graphics card: GTX 1060 latest driver
Short description of error
I have a compositing scene with a broken Viewer. This can be fixed by restarting Blender but I haven't a possible reason investigated yet. It will lose the Viewer preview but saving the result works. The backdrop preview in the compositing window works fine.
Exact steps for others to reproduce the error
I will prepare a scene later.